1. Field of the Invention
The invention concerns a method of nitriding ferrous metal parts, improving their corrosion resistance, in which the parts are treated by immersion for an appropriate time in a bath of molten salts essentially comprising alkali metal cyanates and carbonates.
2. Description of the Prior Art
Salt baths capable of diffusing metalloids, essentially nitrogen and possibly also carbon and sulfur, into the surface layers of ferrous metal parts to improve their resistance to wear and seizing have been known for many years. After using salt baths based on cyanides, the toxicity of which caused implementation problems, baths were used whose active element was essentially the cyanate ion CNO.sup.--, the cations being alkali metals providing chemical stability in combination with a sufficiently low melting point.
Patents FR-A-2 171 993 and FR-A-2 271 307 describe baths of this kind in which the presence of lithium among the alkali metals and small quantities of sulfur-containing substances produce nitrided layers of better quality. FR-A-2 271 307 also describes a method of regenerating the baths by introduction of regenerating salts including, alongside nitrogen-supplying substances, at least one substance having a carboxyl group in its formula, whereby the cyanide concentration is maintained at the trace level, the sulfur acting as a catalyst for the regenerating agent.
As well as improving resistance to wear and seizing, nitriding improves corrosion resistance.
As is well known, the corrosion resistance of nitrided parts can be improved by immersing them for at least ten minutes in oxidizing salt baths including a mixture of alkali metal nitrates and hydroxides at temperatures between 360.degree. C. and 500.degree. C. Patent FR-A-2 525 637 describes a salt bath comprising alkali carbonates, hydroxides and nitrates with a small quantity of an oxygenating alkali metal salt whose oxyreduction potential relative to the reference hydrogen electrode is -1 volt or less. The use of this bath, which further requires the blowing in of air to keep the bath saturated with dissolved oxygen and to limit the concentration of solid particles, substantially increases corrosion resistance.
Nevertheless, the two-stage process: nitriding plus oxidation substantially increases the investment and the cost of manufacture, requiring a duplicated installation of crucibles and additional handling of the parts.
It has therefore been obvious that a single salt bath treatment to obtain the properties of parts subjected to nitriding and then oxidation would have great economical advantages.